Ternary photoinitiator system for addition polymerization

ABSTRACT

Photocurable addition-polymerizable compositions containing a free-radically-polymerizable monomer and a photoinitiator system containing i) an arylidonium salt, ii) a sensitizing compound, and iii) an electron donor having an oxidation potential that is greater than zero and less than or equal to that of p-dimethyoxybenzene (1.32 volts vs. S.C.E.). The compositions cure rapidly and deeply under ultraviolet or visible light.

This is a continuation of application Ser. No. 07/840,880, filed Feb. 25, 1992, now abandoned which is a continuation of application Ser. No. 07/034,065, filed Apr. 2, 1987, now abandoned.

TECHNICAL FIELD

This invention relates to photoinitiator systems for use in addition (free radically-initiated) polymerization.

BACKGROUND ART

Aryliodonium salts have been previously described for use as photoinitiators in addition-polymerizable compositions. References relating to such compositions include U.S. Pat. Nos. 3,729,313, 3,741,769, 3,808,006, 4,228,232, 4,250,053 and 4,428,807; H. J. Timpe and H. Baumann, Wiss. Z. Tech. Hochsch. Leuna-Merseburg, 26, 439 (1984); H. Baumann, B. Strehmel, H. J. Timpe and U. Lammel, J. Prakt. Chem, 326 (3), 415 (1984); and H. Baumann, U. Oertel and H. J. Timpe, Euro. Polym. J., 22 (4), 313 (Apr. 3, 1986).

Mono- and di-ketones have also previously been described for use as photoinitiators in addition-polymerizable compositions. References relating to such compositions include U.S. Pat. Nos. 3,427,161, 3,756,827, 3,759,807 and 4,071,424; U.K. Pat. Specification No. 1,304,112; European Published Pat. Appl. No. 150,952 and Chem. Abs. 95:225704U.

SUMMARY OF THE INVENTION

The present invention provides, in one aspect, an addition-polymerizable composition comprising:

a) free-radically-polymerizable monomer ("monomer"), and

b) photoinitiator system, soluble in said monomer, comprising photochemically effective amounts of

i) diaryliodonium salt "iodonium salt").

ii) sensitizing compound ("sensitizer") capable of absorbing light somewhere within the range of wavelengths between about 300 and about 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and

iii) electron donor compound ("donor"),

said donor being different from said sensitizer and wherein zero <E_(ox) (donor) ≦E_(ox) (p-dimethoxybenzene).

The compositions of the invention provide a very useful combination of cure speed, cure depth and shelf life. They cure well even when loaded with large amounts of fillers, and can be used in a variety of applications including graphic arts imaging (e.g., for color proofing systems, curable inks, or silverless imaging), printing plates (e.g., projection plates or laser plates), photoresists, solder masks, coated abrasives, magnetic media, photocurable adhesives (e.g., for orthodontics) and photocurable composites (e.g., for autobody repair or dentistry).

The invention also provides a method for preparing photopolymerized compositions, and polymerized articles made therefrom.

DETAILED DESCRIPTION OF THE INVENTION

A wide variety of monomers can be photopolymerized using the photoinitiator system of the invention. Suitable monomers contain at least one ethylenically-unsaturated double bond, can be oligomers, and are capable of undergoing addition polymerization. Such monomers include mono-, di- or poly- acrylates and methacrylates such as methyl acrylate, methyl methacrylate, ethyl acrylate, isopropyl methacrylate, n-hexyl acrylate, stearyl acrylate, allyl acrylate, glycerol diacrylate, glycerol triacrylate, ethyleneglycol diacrylate, diethyleneglycol diacrylate, triethyleneglycol dimetbacrylate, 1,3-propanediol diacrylate, 1,3-propanediol dimethacrylate, trimethylolpropane triacrylate, 1,2,4-butanetriol trimethacrylate, 1,4-cyclohexanediol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, pentaerythritol tetramethacrylate, sorbitol hexacrylate, bis[1-(2-acryloxy)]-p-ethoxyphenyldimethylmethane, bis[1-(3-acryloxy-2-hydroxy)]-p-propoxyphenyldimethylmethane, trishydroxyethyl-isocyanurate trimethacrylate; the bis-acrylates and bis-methacrylates of polyethylene glycols of molecular weight 200-500, copolymerizable mixtures of acrylated monomers such as those of U.S. Pat. No. 4,652,274, and acrylated oligomers such as those of U.S. Pat. No. 4, 642,126; unsaturated amides such as methylene bis-acrylamide, methylene bis-methacrylamide, 1,6-hexamethylene bis-acrylamide, diethylene triamine tris-acrylamide and beta-methacrylaminoethyl methacrylate; and vinyl compounds such as styrene, diallyl phthalate, divinyl succinate, divinyl adipate and divinylphthalate. Mixtures of two or more monomers can be used if desired.

The monomer is combined with a three component or ternary photoinitiator system. The first component in the photoinitiator system is the iodonium salt, i.e., a diaryliodonium salt. The iodonium salt should be soluble in the monomer and preferably is shelf-stable (i.e., does not spontaneously promote polymerization) when dissolved therein in the presence of the sensitizer and donor. Accordingly, s election of a particular iodonium salt may depend to some extent upon the particular monomer, sensitizer and donor chosen. Suitable iodonium salts are described in U.S. Pat. Nos. 3,729,313, 3,741,769, 3,808,006, 4,250,053 and 4,394,403, the iodonium salt disclosures of which are incorporated herein by reference. The iodonium salt can be a simple salt (e.g., containing an anion such as Cl⁻, Br⁻, I⁻ or C₄ H₅ SO₃ ⁻) or a metal complex salt (e.g., containing SbF₅ OH⁻ or AsF₆ ⁻). Mixtures of iodonium salts can be used if desired.

Preferred iodonium salts include diphenyliodonium salts such as diphenyliodonium chloride, diphenyliodonium hexafluorophosphate and diphenyliodonium tetrafluoroborate.

The second component in the photoinitiator system is the sensitizer. The sensitizer should be soluble in the monomer, and is capable of light absorption somewhere within the range of wavelengths between about 300 and about 1000 nanometers, more preferably about 400 and about 700 nanometers and most preferably about 400 to about 600 nanometers. The sensitizer is also capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, using the test procedure described in U.S. Pat. No. 3,729,313. Using currently available materials, that test is carried out as follows. A standard test solution is prepared having the following composition:

5.0 parts of a 5% (weight by volume) solution in methanol of 45,000-55,000 molecular weight, 9.0-13.0% hydroxyl content polyvinyl butyral ("Butvar B76", Monsanto)

0.3 parts trimethylolpropane trimethacrylate

0.03 parts 2-methyl-4,6-bis(trichloromethyl)-s-triazine (see Bull. Chem. Soc. Japan, 42, 2924-2930, 1969).

To this solution is added 0.01 parts of the compound to be tested as a sensitizer. The solution is knife-coated onto a 0.05 mm clear polyester film using a knife orifice of 0.05 mm, and the coating is air dried for about 30 minutes. A 0.05 mm clear polyester cover film is carefully placed over the dried but soft and tacky coating with minimum entrapment of air. The resulting sandwich construction is then exposed for three minutes to 161,000 Lux of incident light from a tungsten light source providing light in both the visible and ultraviolet range ("FCH" 650 watt quartz-iodine lamp, General Electric).

Exposure is made through a stencil so as to provide exposed and unexposed areas in the construction. After exposure the cover film is removed and the coating is treated with a finely divided colored powder, such as a color toner powder of the type conventionally used in xerography. If the tested compound is a sensitizer, the trimethylolpropane trimethacrylate monemer will be polymerized by the light-exposed areas by the light-generated free radicals from the 2-methyl-4,6-bis(trichloromethyl)-s-triazine. Since the polymerized areas will be essentially tack-free, the colored powder will selectively adhere only to the tacky, unexposed areas of the coating, providing a visual image corresponding to that in the stencil.

Preferably, in addition to passing the above test, a sensitizer is also selected based in part upon shelf stability considerations. Accordingly, selection of a particular sensitizer may depend to some extent upon the particular monomer, iodonium salt and donor chosen.

Suitable sensitizers are believed to include compounds in the following categories: ketones, coumarin dyes (e.g., ketocoumarins), xanthene dyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, p-substituted aminostyryl ketone compounds, aminotriaryl methanes, merocyanines, squarylium dyes and pyridinium dyes. Ketones (e.g., monoketones or alpha-diketones), ketocoumarins, aminoarylketones and p-substituted aminostyryl ketone compounds are preferred sensitizers. For applications requiring high sensitivity (e.g., graphic arts), it is preferred to employ a sensitizer containing a julolidinyl moiety. For applications requiring deep cure (e.g., cure of highly-filled composites), it is preferred to employ sensitizers having an extinction coefficient below about 1000, more preferably below about 100, at the desired wavelength of irradiation for photopolymerization.

By way of example, a preferred class of ketone sensitizers has the formula:

    ACO(X).sub.b B

where X is CO or CR¹ R² where R¹ and R² can be the same different, and can be hydrogen, alkyl, alkaryl or aralkyl, b is zero, and A and B can be the same or different and can be substituted (having one or more non-interfering substituents) or unsubstituted aryl, alkyl, alkaryl, or aralkyl groups, or together A and B can form a cyclic structure which can be a substituted or unsubstituted cycloaliphatic, aromatic, heteroaromatic or fused aromatic ring.

Suitable ketones of the above formula include monoketones (b=0) such as 2,2-, 4,4- or 2,4-dihydroxybenzophenone, di-2-pyridyl ketone, di-2-furanyl ketone, di-2-thiophenyl ketone, benzoin, fluorenone, chalcone, Michler's ketone, 2-fluoro-9-fluorenone, 2-chlorothioxanthone, acetophenone, benzophenone, 1- or 2-acetonaphthone, 9-acetylanthracene, 2-, 3- or 9-acetylphenanthrene, 4-acetylbiphenyl, propiophenone, n-butyrophenone, valerophenone, 2-, 3- or 4-acetylpyridine, 3-acetylcoumarin and the like. Suitable diketones include aralkyldiketones such as anthraquinone, phenanthrenequinone, o-, m- and p-diacetylbenzene, 1,3-, 1,4-, 1,5-, 1,6-, 1,7- and 1,8-diacetylnaphthalene, 1,5-, 1,8- and 9,10-diacetylanthracene, and the like. Suitable α-diketones (b=1 and x=CO) include 2,3-butanedione, 2,3-pentanedione, 2,3-hexanedione, 3,4-hexanedione, 2,3-heptanedione, 3,4-heptanedione, 2,3-octanedione, 4,5-octanedione, benzil, 2,2'- 3 3'- and 4,4'-dihydroxylbenzil, furil, di-3,3'-indolylethanedione, 2,3-bornanedione (camphorquinone), biacetyl, 1,2-cyclohexanedione, 1,2-naphthaquinone, acenaphthaquinone, and the like.

Preferred ketocoumarins and p-substituted aminostyryl ketone compounds are listed in TABLE II, below.

The third component in the photoinitiator system is the electron donor. A wide variety of donors can be employed. The donor is soluble in the monomer, and should meet the oxidation potential (E_(ox)) limitation discussed in more detail below. Preferably, the donor also is selected based in part upon shelf stability considerations. Accordingly, a selection of a particular donor may depend in part on the monomer, iodonium salt and sensitizer chosen. Suitable donors are capable of increasing the speed of cure or depth of cure of a composition of the invention upon exposure to light of the desired wavelength. Also, the donor has an E_(ox) greater than zero and less than or equal to E_(ox) (p-dimethoxybenzene). Preferably E_(ox) (donor) is between about 0.5 and 1 volts vs. a saturated calomel electrode ("S.C.E."). E_(ox) (donor) values can be measured experimentally, or obtained from references such as N. L. Weinburg, Ed., Technique of Electroorganic Synthesis Part II Techniques of Chemistry, Vol. V (1975), and C. K. Mann and K. K. Barnes, Electrochemical Reactions in Nonaqueous Systems (1970).

Preferred donor S include amines (including aminoaldehydes and aminosilanes), amides (including phosphoramides), ethers (including thioethers), ureas (including thioureas), ferrocene, sulfinic acids and their salts, salts of ferrocyanide, ascorbic acid and its salts, dithiocarbamic acid and its salts, salts of xanthates, salts of ethylene diamine tetraacetic acid and salts of tetraphenylboronic acid. The donor can be unsubstituted or substituted with one or more non-interfering substituents. Particularly preferred donors contain an electron donor atom such as a nitrogen, oxygen, phosphorus, or sulfur atom, and an abstractable hydrogen atom bonded to a carbon or silicon atom alpha to the electron donor atom.

Preferred amine donor compounds include alkyl-, aryl-, alkaryl- and aralkyl-amines such as methylamine, ethylamine, propylamine, butylamine, triethanolamine, amylamine, hexylamine, 2,4-dimethylaniline, 2,3-dimethylaniline, o-, m- and p-toluidine, benzylamine, aminopyridine, N,N'-dimethylethylenediamine, N,N'-diethylethylenediamine, N,N'-dibenzylethylenediamine, N,N'-diethyl-1,3-propanediamine, N,N'-diethyl-2-butene-1,4-diamine, N,N'-dimethyl-1,6-hexanediamine, piperazine, 4,4'-trimethylenedipiperidine, 4,4'-ethylenedipiperidine, p-N N-dimethyl-aminophenethanol and p-N-dimethylamxnobenzonitrile; aminoaldehydes such as p-N,N-dimethylaminobenzaldehyde, p-N,N-diethylaminobenzaldehyde, 9-julolidine carboxaldehyde and 4-morpholinobenzaldehyde; and aminosilanes such as trimethylsilylmorpholine, trimethylsilylpiperidine, bis(dimethylamino)diphenylsilane, tris(dimethylamino)methylsilane, N,N-diethylaminotrimethylsilane, tris(dimethylamino)phenylsilane, tris(methylsilyl)amine, tris(dimethylsilyl)amine, bis(dimethylsilyl)amine, N,N-bis(dimethylsilyl)aniline, N-phenyl-N-dimethylsilylaniline and N,N-dimethyl-N-dimethylsilylamine. Tertiary aromatic alkylamines, particularly those having at least one electron-withdrawing group on the aromatic ring, have been found to provide especially good shelf stability. Good shelf stability has also been obtained using amines that are solids at room temperature. Good photographic speed has been obtained using amines that contain one or more julolidinyl moieties.

Preferred amide donor compounds include N,N-dimethylacetamide, N,N-diethylacetamide, N-methyl-N-phenylacetamide, hexamethylphosphoramide, hexaethylphosphoramide, hexapropylphosphoramide, trimorpholinophosphine oxide and tripiperidinophosphine oxide.

Suitable ether donor compounds include 4,4'-dimethoxybiphenyl, 1,2,4-trimethoxybenzene and 1,2,4,5-tetramethoxybenzene.

Suitable urea donor compounds include N,N'-dimethylurea, N,N-dimethylurea, N,N'-diphenylurea, tetramethylthiourea, tetraethylthiourea, tetra-n-butylthiourea, N,N-di-n-butylthiourea, N,N'-di-n-butylthiourea, N,N-diphenylthiourea and N,N'-diphenyl-N,N'-diethylthiourea.

The three components of the photoinitiator system are present in "photochemically effective amounts", that is, amounts of each component sufficient to enable the monomer to undergo photochemical gelation or hardening upon exposure to light of the desired wavelength. Preferably, for every 10 parts of monomer, a composition of the invention contains about 0.005 to about 10 parts (more preferably about 0.1 to about 4 parts) each of iodonium salt, sensitizer and donor. The amounts of each component are independently variable and thus need not be equal, with larger amounts generally providing faster cure, but shorter shelf life. Sensitizers with high extinction coefficients (e.g., above about 10,000) at the desired wavelength of irradiation for photopolymerization generally are used in reduced amounts.

The compositions of the invention can contain a wide variety of adjuvants depending upon the desired end use. Suitable adjuvants include solvents, diluents, resins, binders, plasticizers, pigments, dyes, inorganic or organic reinforcing or extending fillers (at preferred amounts of about 10% to about 90% by weight, based on the total weight of the composition), thixotropic agents, indicators, inhibitors, stabilizers, UV absorbers, medicaments (e.g., leachable fluorides) and the like. The amounts and types of such adjuvants, and their manner of addition to a composition of the invention will be familiar to those skilled in the art.

The compositions of the invention can be cured using a variety of methods. It is convenient to employ light sources that emit ultraviolet or visible light such as quartz halogen lamps, tungsten-halogen lamps, mercury arcs, carbon arcs, low-, medium-, and high-pressure mercury lamps, plasma arcs, light emitting diodes and lasers. Electron beam ("E-beam") irradiation and other curing devices that do not depend on light emission can also employed. In general, heat or an inert atmosphere will accelerate cure.

The following examples are offered to aid in understanding the invention and are not to be construed as limiting the scope thereof. Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1

Three stock solutions were prepared from 0.25 parts camphorquinone (CPQ), 50 parts triethyleneglycol dimethacrylate (TEGDMA) and 50 parts bisphenol A diglycidyl ether dimethacrylate (BisGMA). 0.50 Part diphenyliodonium hexafluorophosphate (φ₂ I⁺ PF₆ ⁻) was added to the first solution. 0.25 Part sodium p-toluenesulfinate (STS) was added to the second solution. 0.50 Part φ₂ I⁺ PF₆ ⁻ and 0.25 part STS were added to the third solution. Each solution was poured into a 6 mm diameter "Teflon" mold to a depth of 2.5 mm, covered with polyester film and irradiated for 10 seconds using a handheld visible light curing lamp ("Visilux", 3M) whose lightguide output end was placed directly on the polyester film.

The solutions containing only CPQ and φ₂ I⁺ PF₆ ⁻ or CPQ and STS formed a soft gel. The solution containing CPQ, φ₂ I⁺ PF₆ ⁻ and STS hardened to a solid having a Barcol hardness of 40 (ASTM D-2583) on both its top and bottom surfaces.

In a further experiment, three stock solutions were prepared from 11.85 parts each of the above monomers, 76 parts filler, and 0.25 part CPQ. 0.25 Part φ₂ I⁺ PF₆ ⁻ was added to the first solution. 0.25 Part N,N-dimethyl-aminophenethyl alcohol ("D-1") was added to the second solution. 0.25 Part φ₂ I⁺ PF₆ ⁻ and 0.25 part D-1 were added to the third solution. Each solution was cured in a mold as described above, but using a 6 mm deep mold and a 20 second cure time. The solution containing only φ₂ I⁺ PF₆ ⁻ did not cure. The solution containing only D-1 had top and bottom Barcol hardness values of 56 and 2, respectively. The solution containing both φ₂ I⁺ PF₆ ⁻ and D-1 had top and bottom Barcol hardness values of 60 and 30, respectively.

The above data illustrates that an increased degree of polymerization and depth of cure can be obtained using a composition of the invention.

EXAMPLE 2

Equimolar amounts of a variety of donors were added to monomer stock solutions containing 50 parts trimethylolpropane trimethacrylate, 50 parts 1,4-butanediol dimethacrylate, 0.25 part CPQ and optionally 0.5 part of the iodonium salt φ₂ I⁺ PF₆ ⁻.

The resulting solutions were irradiated with visible light at an intensity of 60 mW/cm² (as measured by a United Detector Technology Model 351 portable photometer/radiometer) at 400-500 nm. The solutions were stirred using a glass rod and the time required to reach the gelation point was recorded. Set out below in TABLE I are the run number, donor compound, E_(ox) (donor) VS. S.C.E., weight percent donor, and gel times for solutions prepared with and without the iodonium salt. The donors are listed in TABLE I in order of generally decreasing oxidation potential.

                                      TABLE I                                      __________________________________________________________________________                               Gel time, seconds                                                                    Sensitizer/                                    Run              E.sub.ox                                                                            %   Sensitizer/                                                                          donor/                                         No.                                                                               Donor         (donor)                                                                             donor                                                                              donor iodonium salt                                  __________________________________________________________________________     1  control            0   >200  190                                            2  acetonitrile  2.60 0.124                                                                              >200  >190                                           3  nitrobenzene       0.373                                                                              >200  >190                                           4  methylethylketone  0.218                                                                              >200  >190                                           5  2,5-dimethyl-2,4-hexadiene                                                                   2.10 0.334                                                                              >200  >190                                           6  ethylmethylthioacetate                                                                       1.70 0.407                                                                              >200  >190                                           7  p-bromothioanisole                                                                           1.60 0.615                                                                              >200  >190                                           8  3,3'-dimethoxybiphenyl                                                                       1.60 0.649                                                                              >200  >190                                           9  tetrahydrofuran                                                                              1.60 0.220                                                                              >200  >190                                           10 hexaethylbenzene                                                                             1.49 0.492                                                                              >200  >190                                           11 methoxyphenylphenylether                                                                          0.604                                                                              >200  188                                            12 p-dimethoxybenzene                                                                           1.34 0.418                                                                              205   160                                            13 N,N-dimethylacetamide                                                                        1.32 0.264                                                                              204   150                                            14 phenylacetate 1.30 0.413                                                                              >200  >190                                           15 n-propylamine 1.30 0.200                                                                              90    24                                             16 aniline       1.28 0.282                                                                              >200  >190                                           17 1,3-dibutylthiourea                                                                               0.570                                                                              >200  137                                            18 tetramethylurea    0.352                                                                              94    101                                            19 tetrabutylthiourea 0.909                                                                              38    29                                             20 dipentylamine 1.22 0.477                                                                              159   19                                             21 1,2,4-trimethoxybenzene                                                                      1.12 0.509                                                                              >225  55                                             22 hexamethylphosphoramide                                                                      1.00 0.543                                                                              80    50                                             23 tripiperdinophosphine oxide                                                                  1.00 0.907                                                                              52    40                                             24 trimethylsilylmorpholine                                                                          0.483                                                                              112   21                                             25 N,N-dimethylbenzylamine                                                                      1.00 0.410                                                                              18    8                                              26 tris-dimethylsilylamine                                                                           0.580                                                                              108   32                                             27 triethanolamine                                                                              0.96 0.452                                                                              17    6                                              28 tris(dimethylamino)-                                                                              0.719                                                                              15    9                                                 phenylsilane                                                                29 triphenylamine                                                                               0.86 0.737                                                                              >200  >190                                           30 triphenylphosphine 0.794                                                                              >200  172                                            31 p-dimethylamino-                                                                             0.70 0.452                                                                              13    11                                                benzaldehyde                                                                32 N,N-dimethyl-p-toluidine                                                                     0.65 0.410                                                                              14    7                                              33 N,N-dimethylaminophenethyl                                                                   0.65 0.500                                                                              13    8                                                 alcohol                                                                     __________________________________________________________________________

The above data illustrates an increased cure rate is obtained using a composition of the invention, and demonstrates the advantage of using donors whose E_(ox) value is less than or equal to that of p-dimethoxybenzene and that have an abstractable hydrogen atom on a carbon or silicon atom alpha to the donor atom.

EXAMPLE 3

Solutions similar to those of U.S. Pat. No. 4,228,232 (Rousseau) were prepared from 4.3 parts pentaerythritol tetraacrylate, 5.6 parts "Oligomer P-II" (Rousseau, Col. 11, 61% by weight in MEK), 0.30 parts triethylamine, 14.9 parts of a dispersion containing 4.1% Pigment Red 48 (CI 15865 ) and 8.2%. polyvinyl formal resin ("Formvat 12/85" Monsanto) in n-propanol-water azeotrope, and 74.2 parts n-propanol-water azeotrope. As was the case in Rousseau, the amount of triethylamine was just sufficient to neutralize the acid in oligomer P-II, and thus the triethylamine did not serve as a donor. To 5 parts of the above solutions were added 0.02 part φ₂ I⁺ PF₆ ⁻, varying amounts of various sensitizers, and optionally 0.015 part of the donor compound 9-julolidine carboxyaldehyde. The solutions were then coated with wire-wound rods onto grained and anodized aluminum at coating weights of 1-2 g/m² and dried at 66° C. for 2 minutes. The resulting photosensitive printing plates were topcoated with 5% aqueous polyvinyl alcohol containing 0.26 parts surfactant ("Triton X-100" Rohm and Haas) as a coating aid at coating weights of 1-2 g/m². The dried plates were exposed through a √2 density increment, 21 step sensitivity guide (Stouffer Graphic Arts) for 2 sec. using a 172,000 Lux tungsten light source at a distance of 25.4 cm ("Model 70" Transparency Maker, 3M). The exposed plates were developed with an aqueous solution of 4% n-propanol, 2% sodium metasilicate and 0.06% surfactant ("Dowfax 2AL", Dow Chemical Company). Set out below in TABLE II are the run number, the type and amount of sensitizer and its λ_(max) value, a statement ("yes" or "no") indicating whether or not the optional donor was added, and the number of solid steps retained after exposure and development. Higher step values indicate greater sensitivity. Exposure values (in ergs/cm² required to form one solid step; last column in TABLE II) for many of the plates were also measured by exposing the plates to coherent light using an argon-ion laser (488 nm), or by exposing the plates to incoherent light at the absorption maximum of the sensitizer using a one Kw high pressure mercury-xenon light source directed through a variable frequency monochromator having a 20 nm bandpass. The monochromator output was measured using a radiometer. Numerically lower exposure values indicate increased sensitivity.

From the data in TABLE II it can be seen that plates containing iodonium salt, sensitizer, and donor are more sensitive than plates containing only iodonium salt and sensitizer. An exposure value of 450 ergs/cm² at 488 nm (argon-ion laser) was obtained using a combination of diphenyliodonium salt, ketocoumarin sensitizer, and 9-julolidine carboxaldehyde (Run 8). For comparison, the relatively sensitive laser-imageable compositions of Rousseau Exs. 18 and 19 (cols. 18-19) have an exposure value at 488 nm of only 3,200 ergs/cm².

                                      TABLE II                                     __________________________________________________________________________                                                 Sensitivity,                       Run                                                                               Sensitizer                       Add'l                                                                              Solid                                                                              ergs/cm.sup.2                      No.                                                                               Type                   Amount                                                                              λ.sub.max, nm                                                                donor                                                                              steps.sup.(a)                                                                      (wavelength)                       __________________________________________________________________________     1  3-(p-dimethylaminocinnamoyl)-7-dimethyl-                                                              0.005                                                                               475  no  10   10,000.sup.(b)                       aminocoumarin                            (488 nm)                           2  3-(p-dimethylaminocinnamoyl)-7-dimethyl-                                                              0.005                                                                               475  yes 13   3,000.sup.(b)                        aminocoumarin                            (488 nm)                           3  3-(p-diethylaminocinnamoyl)-7-dimethyl-                                                               0.005                                                                               480  no  13   3,000.sup.(b)                        aminocoumarin                            (488 nm)                           4  3-(p-diethylaminocinnamoyl)-7-dimethyl-                                                               0.005                                                                               480  yes 15     900.sup.(b)                        aminocoumarin                            (488 nm)                           5  9'-julolidine-4-piperidinoacetophenone                                                                0.008                                                                               450  no  15    900                                                                          (450 nm)                           6  9'-julolidine-4-piperidinoacetophenone                                                                0.008                                                                               450  yes 18    300                                                                          (450 nm)                           7  9-(4-diethylaminocinnamoyl)-1,2,4,5-tetrahydro-                                                       0.005                                                                               500  no  14   1,400.sup.(b)                        3H,6H,10H[1]benzopyrano[6,7,8-i,j]quinolizine-                                                                          (488 nm)                              10-one                                                                      8  9-(4-diethylaminocinnamoyl)-1,2,4,5-tetrahydro-                                                       0.005                                                                               500  yes 16     450.sup.(b)                        3H,6H,10H[1]benzopyrano[6,7,8-i,j]quinolizine-                                                                          (488 nm)                              10-one                                                                      9  9-(4-dicyanoethylaminocinnamoyl)-1,2,4,5-tetra-                                                       0.005                                                                               495  no  8   .sup.(c)                              hydro-3H,6H,10H[1]benzopyrano[6,7,8-i,j]-                                      quinolizine-10-one                                                          10 9-(4-dicyanoethylaminocinnamoyl)-1,2,4,5-tetra-                                                       0.005                                                                               495  yes 12  .sup.(c)                              hydro-3H,6H,10H[1]benzopyrano[6,7,8-i,j]-                                      quinolizine-10-one                                                          11 2,3-bis(9'-julolidine)cyclopentanone                                                                  0.005                                                                               525  no  12  3,200                                                                          (514 nm)                           12 2,3-bis(9'-julolidine)cyclopentanone                                                                  0.005                                                                               525  yes 14  1,400                                                                          (514 nm)                           13 9-ethoxycarbonyl-1,2,4,5-tetrahydro-3H,6H,10H-                                                        0.008                                                                               435  no  16    500                                 [1]benzopyrano[6,7,8-i,j]quinolizine-10-one                                                                             (435 nm)                           14 9-ethoxycarbonyl-1,2,4,5-tetrahydro-3H,6H,10H-                                                        0.008                                                                               435  yes 18    250                                 [1]benzopyrano[6,7,8-i,j]quinolizine-10-one                                                                             (435 nm)                           15 2-(4'-diethylaminobenzylidine)-1-indanone                                                             0.008                                                                               439  no  12  .sup.(c)                           16 2-(4'-diethylaminobenzylidine)-1-indanone                                                             0.008                                                                               439  yes 14  .sup.(c)                           17 9-acetyl-1,2,4,5-tetrahydro-3H,6H,10H[1]benzo-                                                        0.008                                                                               450  no  13  3,200                                 pyrano[6,7,8-i,j]quinolizine-10-one      (450 nm)                           18 9-acetyl-1,2,4,5-tetrahydro-3H,6H,10H[1]benzo-                                                        0.008                                                                               450  yes 15  1,500                                 pyrano[6,7,8-i,j]quinolizine-10-one      (450 nm)                           19 5,10-diethoxy-12,16,17-trichloroviolanthrene                                                          0.005                                                                               540  no   3  .sup.(c)                           20 5,10-diethoxy-12,16,17-trichloroviolanthrene                                                          0.005                                                                               540  yes  5  .sup.(c)                           __________________________________________________________________________      Notes to TABLE II:                                                             .sup.(a) Exposure using tungsten light source.                                 .sup.(b) Exposure using 488 nm argonion laser.                                 .sup.(c) Not determined.                                                 

EXAMPLE 4

Coating formulation was prepared from the following ingredients:

    ______________________________________                                                            % Solids                                                    ______________________________________                                         acrylamide           43.3                                                      N,N'-methylenebisacrylamide                                                                         4.3                                                       polyvinyl alcohol    51.9                                                      (m.w. 2000, 75% hydrolyzed)                                                                         0.5                                                       surfactant ("Triton X-100")                                                    ______________________________________                                    

made up to 11.5% solids in a 1/1 v/v acetonitrile/water mixture. Using a red safelight, coating samples were prepared by combining 25 ml portions of the above stock solution with 0.01 g of the sensitizer and optionally adding 0.1 g, φ₂ I⁺ PF₆ ⁻ and/or 0.1 g STS. The samples were coated onto gelatin-subbed polyester film using a #18 wire wound rod, dried with a heat gun, then oven-dried for 2 minutes at 60° C. The coated films were exposed under vacuum through a 21 step sensitivity guide, using a tungsten ("Model 70" Transparency Maker, 3M) or ultraviolet (2 Kw Berkey Ascor, Berkey Technical Company) light source. The exposed samples were developed using a 3/20, v/v water/methanol solvent mixture. Relative speed was determined by the number of steps (average of 3 samples) remaining after development. Set out below in TABLE III are the results for the samples exposed to visible light, and set out below in TABLE IV are the results for the samples exposed to ultraviolet light. Each exposure was 30 sec. in the visible region or 60 sec. in the ultraviolet region except as noted.

                                      TABLE III                                    __________________________________________________________________________     Visible Light Sensitivity Enhancement                                                                        Solid steps                                                                                 Sensitizer/                         Run                      Sensitizer                                                                          Sensitizer/                                                                          Sensitizer/                                                                           donor/                              No.                                                                               Sensitizer            λ.sub.max, nm                                                                donor iodonium salt                                                                         iodonium sal                        __________________________________________________________________________     1  methylene blue        661  .sup.(c)                                                                             3      12                                  2  toluidine blue        626  6     .sup.(c)                                                                              16                                  3  rose bengal           548  .sup.(d)                                                                             .sup.(c)                                                                              16                                  4  phenosafranine        520  .sup.(d)                                                                             .sup.(c)                                                                               9                                  5  1,3-bis(4-dimethylaminobenzilidene)acetone.sup.(a)                                                   434  .sup.(d)                                                                             11     17                                  6  tris(bipyridyl)ruthenium(+2) chloride                                                                453  .sup.(d)                                                                             .sup.(c)                                                                              12                                  7  crystal violet.sup.(b)                                                                               593  .sup.(d)                                                                             .sup.(c)                                                                              10                                  8  eosin yellow          517  .sup.(c)                                                                             4      12                                  9  3,3'-dimethylthiocarbocyanine iodide.sup.(a)                                                         553  .sup.(d)                                                                             .sup.(c)                                                                              12                                  __________________________________________________________________________      Notes to TABLE III:                                                            .sup.(a) 5 sec. exposure.                                                      .sup.(b) 60 sec. exposure.                                                     .sup.(c) Image lost during development.                                        .sup.(d) No image formed.                                                

                                      TABLE IV                                     __________________________________________________________________________     Ultraviolet Light Sensitivity Enhancement                                                                   Solid steps                                                                                 Sensitizer/                          Run                     Sensitizer                                                                          Sensitizer/                                                                          Sensitizer/                                                                           donor/                               No.                                                                               Sensitizer           λ.sub.max, nm                                                                donor iodonium salt                                                                         iodonium salt                        __________________________________________________________________________     1  2,5-bis(cinamylidene)cyclopentanone                                                                 400  .sup.(b)                                                                             .sup.(b)                                                                               8                                   2  4'-methoxybenzylidene-4-nitro-acetophenone                                                          356  .sup.(b)                                                                             .sup.(b)                                                                              11                                   3  2-(4-dimethylaminobenzilidene)-                                                                     377  .sup.(b)                                                                             .sup.(c)                                                                               5                                      dimethylmalonate                                                            4  Michler's ketone.sup.(a)                                                                            355  .sup.(b)                                                                             10     15                                   5  2-chlorothioxanthone 387  .sup.(b)                                                                             .sup.(c)                                                                              11                                   __________________________________________________________________________      Notes to TABLE IV:                                                             .sup.(a) 15 Second exposure at 40% power.                                      .sup.(b) No image formed.                                                      .sup.(c) Image lost during development.                                  

The above data illustrate the combination of an iodonium salt, sensitizer and donor can increase imaging speed by one to two orders of magnitude compared to compositions containing only sensitizer and donor or only sensitizer and iodonium salt.

EXAMPLE 5

To illustrate the effect of E_(ox) (donor) upon cure speed, a series of compositions was evaluated as follows. A monomer stock solution was prepared from 10% pentaerythritol tetraacrylate in 4/1, w/w, acetonitrile/water. To 3 ml portions of this solution in 13×100 mm "Pyrex" test tubes were added about 0.02 g of φ₂ I⁺ PF₆ ⁻ and/or a donor compound and enough sensitizer to give an optical density of between 1 and 2, as evaluated visually. The solutions were purged with N₂ for 2 minutes before and continuously during light irradiation. The light source was a Kodak "Carousel" Projector lamp equipped with a 440 nm filter. Relative speed was determined by measuring gelation time.

Set out below in TABLE V are the run number, the sensitizers and their λ_(max) values, the donor compounds and their E_(ox) values VS. S.C.E., and the gelation times for solutions containing iodonium salt plus sensitizer, sensitizer plus donor, or iodonium salt plus sensitizer plus donor.

                                      TABLE V                                      __________________________________________________________________________                                        Gel time, sec.                                                                 Iodonium                                                                            Sensi-                                                                             Iodonium salt/                     Run                                                                               Sensitizer                      salt/                                                                               tizer/                                                                             sensitizer/                        No.                                                                               Identity                λ.sub.max                                                                  Donor.sup.(a)                                                                       sensitizer                                                                          Donor                                                                              donor                              __________________________________________________________________________     1  methylene blue          661                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           10                                 2  thionin                 598                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           180                                3  thionin                 598                                                                               FC   .sup.(c)                                                                            .sup.(c)                                                                           <5                                 4  phenosafranine          520                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           15                                 5  rose bengal             548                                                                               STS  .sup.(b)                                                                            30  <5                                 6  fluorescein             491                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           15                                 7  crystal violet          588                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           >300                               8  crystal violet          588                                                                               FC   .sup.(c)                                                                            .sup.(c)                                                                           120                                9  malachite green         614                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           >300                               10 malachite green         614                                                                               FC   .sup.(c)                                                                            .sup.(c)                                                                           60                                 11 3,3'-dimethylthiocarbocyanine iodide                                                                   553                                                                               STS  60   .sup.(b)                                                                           30                                 12 2,6-bis(4-dimethylaminostyryl)-1-methyl-                                                               490                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           .sup.(b)                              pyridinium iodide                                                           13 2,6-bis(4-dimethylaminostyryl)-1-methyl-                                                               490                                                                               FC   .sup.(c)                                                                            .sup.(c)                                                                           90                                    pyridinium iodide                                                           14 tris(bipyridyl)ruthenium(+2) chloride                                                                  453                                                                               STS  60   60  <10                                15 1-methylaminoanthraquinone                                                                             502                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           45                                 16 1,2,2-tricyano-1-(4-dimethylaminophenyl)ethylene                                                       525                                                                               STS  .sup.(b)                                                                            .sup.(b)                                                                           .sup.(b)                           17 1,2,2-tricyano-1-(4-dimethylaminophenyl)ethylene                                                       525                                                                               FC   .sup.(c)                                                                            .sup.(c)                                                                           90                                 __________________________________________________________________________      Notes to TABLE V:                                                              .sup.(a) STS = sodium ptoluenesulfinate (E.sub.ox =0.76); FC =                 ferrocyanide (E.sub.ox = 0.2).                                                 .sup.(b) No reaction.                                                          .sup.(c) Not determined.                                                 

The above data illustrates that when E_(ox) (donor) is decreased, cure speed generally increases (see also TABLE I).

EXAMPLE 6

A polyester cloth backing (woven, spun polyester cloth, 0.03 g/cm², Milliken) was impregnated with resin by saturating the backing with an ultraviolet-light-curable resin mixture made by combining 75 parts epoxy-acrylate resin ("Novacure 3500", Interez), 15 parts pentaerythritol triacrylate, 9 parts n-vinyl pyrrolidone and 1 part α,α-dimethoxy-α-phenyl acetophenone, and curing the resin under ultraviolet light in air using an energy density of 0.3 J/cm².

A coated abrasive binder resin was prepared from a 50:50 mixture of the triacrylate of tris-hydroxyethylisocyanurate and the triacrylate of trimethylolpropane, filled to 50% with calcium carbonate. 0.25 Part each of φ₂ I⁺ PF₆ ⁻, CPQ and D-1 were mixed into the binder resin.

The resin-impregnated backing was knife-coated with the binder resin at a coating thickness of 0.1 mm, then drop-coated with grade 50 Al₂ O₃ mineral. The binder was cured under nitrogen in 5 to 10 seconds using a high intensity visible light source (Model F440 with 4V678 lamp, Fusion Systems) operated at a distance of about 15 cm. Microscopic examination of the cured abrasive showed that cure took place throughout the binder, even underneath individual mineral granules. By comparison, if the photoinitiator system was excluded from the resin and a 5 Mrad dose of E-beam irradiation (250 Kv acceleration potential) was employed to effect cure, pools of wet uncured resin remained under individual mineral granules and the granules were poorly adhered on the backing.

A sample of the coated abrasive was size coated with the same resin system, using a coating weight just sufficient to coat the mineral granules. The size coat was cured under the visible light source used to cure the make coat. The resulting coated abrasive was evaluated using a reciprocating grinding apparatus ("rocker drum") on a 6 mm thick 1018 carbon steel workpiece. After 500 cycles, no shelling was observed and an average of 0.77 g of steel was removed. A comparison abrasive sample was prepared using a make coat that contained the same photoinitiator system (cured using visible light), and a size coat that did not contain the photoinitiator system (cured using E-beam). The comparison abrasive shelled after only 20 cycles and removed only 0.02 g of steel.

Comparable results were obtained when sensitizers such as benzil, 2-chlorothioxanthone and fluorenone were substituted for camphorguinone. Improved uncured resin shelf life and ambient light stability were obtained when donor compounds such as ethyl p-dimethylaminobenzoate, p-dimethylaminobenzaldehyde and p-dimethylaminobenzonitrile were substituted for p-dimethylaminophenethyl alcohol.

The above-described abrasive binder system should be a useful substitute for standard phenolic-based binders, and could offer reduced energy consumption and higher throughput during manufacture.

EXAMPLE 7

A visible-light-curable orthodontic bracket adhesive was prepared by mixing together 197 parts BisGMA, 197 parts TEGDMA, 1,618 parts silane-treated microparticles (Example 1, U.S. Pat. No. 4,503,169), 33 parts pyrogenic silica ("R-972", DeGussa), 2 parts φ₂ I⁺ PF₆ ⁻, 1 part CPQ and 3 parts triethanolamine to yield a paste-like composition. A bovine incisal tooth was potted in a circular acrylic cylinder, and the labial surface of the tooth cleaned with a pumice slurry, rinsed, dried and etched with gelled 37% phosphoric acid for 1 minute. The tooth was rinsed with distilled water for 45 seconds, air dried, coated on its etched surface with a methacrylatebased primer ("Light-Curable Enamel Bond", 3M), blown with air to level the primer, and irradiated for 20 seconds with a handheld curing light ("Visilux", 3M).

A thin film of the above-described orthodontic bracket adhesive was applied to the entire back surface of a standard stainless steel orthodontic bracket (4.0×4.0 mm, 100 mesh backing). The bracket was then pressed firmly against the etched, primed bovine tooth. The lightguide output end was oriented orthogonally with respect to the face of the bracket, at a distance of about 3 mm, and the light energized for 20 seconds.

Fifty brackets were bonded to teeth using the above procedure. The resulting cured bracket-tooth assemblies were stored for various periods of time, then clamped in the fixed jaw of an "Instron" tensile tester (Instron Engineering Corporation). The movable jaw of the tester was fastened to the bracket using a length of orthodontic wire. The shear force required to cause bond failure was measured at a jaw separation rate of 2 mm/minute, and an average value determined for five samples at each storage time. Set out below in TABLE VI are the storage times and measured average shear bond strengths for the above-described adhesive, along with comparison values for a conventional two-part chemical cure orthodontic adhesive ("Concise" No. 1960, 3M) and a commercially available one-part light cure orthodontic adhesive ("Heliosit", Vivadent, cured according to the manufacturer's instructions).

                  TABLE VI                                                         ______________________________________                                                Average shear bond strength, kg/cm.sup.2                                Storage time                                                                            Adhesive of EXAMPLE 7                                                                           Concise   Heliosit                                   ______________________________________                                         30 seconds                                                                              103.3            .sup.(c)  53.6                                        2 minutes                                                                              112.3            .sup.(c)  68.5                                        5 minutes                                                                              102.9             82.0     77.2                                       10 minutes                                                                              99.1             105.6     80.0                                        1 hour.sup.(a)                                                                         142.9            150.0     86.1                                       24 hours.sup.(a) (b)                                                                    134.9            115.6     93.4                                        3 months.sup.(a)                                                                       160.1            .sup.(c)  .sup.(c)                                    6 months.sup.(a)                                                                       186.5            .sup.(c)  .sup.(c)                                   12 months.sup.(a)                                                                       172.2            .sup.(c)  .sup.(c)                                   18 months.sup.(a)                                                                       161.6            .sup.(c)  .sup.(c)                                   ______________________________________                                          Notes to TABLE VI:                                                             .sup.(a) Stored in 37° C. distilled water.                              .sup.(b) Average of 10 samples, prepared using two different batches of        adhesive.                                                                      .sup.(c) Not determined.                                                 

The commercial chemical cure adhesive requires at least 5 minutes waiting time before archwire application. The commercial light cure adhesive also requires some waiting time before archwire application, and its ultimate strength is believed to be insufficiently high to prevent occasional bracket adhesion failure. In contrast, the adhesive made from the photoinitiator system of this invention exhibits rapid strength buildup and high ultimate bond strength. It should enable immediate archwire application, and reduce the incidence of early bracket adhesion failure.

Various modifications and alterations of this invention will be apparent to those skilled in the without departing from the scope and spirit of this invention. It should be understood that this invention is not limited to the illustrative embodiments set forth herein. 

We claim:
 1. An addition-polymerizable composition comprising:a) free-radically-polymerizable monomer, and b) photoinitiator system, soluble in said monomer, comprising photochemically effective amounts ofi) aryliodonium salt, ii) sensitizing compound capable of absorbing light within the range of wavelengths between about 300 and about 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and iii) electron donor compound, said donor being different from said sensitizing compound wherein the oxidation potential of said donor is greater than zero and less than or equal to p-dimethoxybenzene vs. a saturated calomel electrode.
 2. A composition according to claim 1, wherein said aryliodonium salt comprises a diphenyliodonium simple salt or diphenyliodonium metal complex salt.
 3. A composition according to claim 1, wherein said range of wavelengths is about 400 to about 700 nanometers.
 4. A composition according to claim 1, wherein said range of wavelengths is about 400 to about 600 nanometers.
 5. A composition according to claim 1, wherein said sensitizing compound is selected from the group consisting of ketones, coumarin dyes, xanthene dyes, acridine dyes, thiazole dyes, thiazine dyes, oxazine dyes, azine dyes, aminoketone dyes, porphyrins, aromatic polycyclic hydrocarbons, p-substituted aminostyryl ketone compounds, aminotriaryl methanes, merocyanines squarylium dyes and pyridinium dyes.
 6. A composition according to claim 1, wherein said sensitizing compound comprises a ketone, ketocoumarin, aminoarylketone or p-substituted aminostyryl ketone compound.
 7. A composition according to claim 1, wherein said sensitizing compound contains at least one julolidinyl moiety.
 8. A composition according to claim 1, wherein said sensitizing compound comprises an α-diketone having an extinction coefficient below about 1000 at the wavelength at which said composition is irradiated when photopolymerized.
 9. A composition according to claim 1, wherein said sensitizing compound comprises camphorquinone.
 10. A composition according to claim 1, wherein said sensitizing compound has the formula:

    ACO(X).sub.b B

where X is CO or CR¹ R², where R¹ and R² are the same or different, and are hydrogen, alkyl, alkaryl or aralkyl, b is zero or 1, and A and B are the same or different and are substituted or unsubstituted aryl, alkyl, alkaryl or aralkyl groups, or together A and B form a cyclic structure which is a substituted or unsubstituted cycloaliphatic, aromatic, heteroaromatic or fused aromatic ring.
 11. A composition according to claim 1, wherein the oxidation potential of the donor is between about 0.5 and 1 volts vs. a saturated calomel electrode.
 12. A composition according claim 1 wherein said donor is selected from the group consisting of amines, amides, ethers, ureas, ferrocene, sulfinic acids and their salts, salts of ferrocyanide, ascorbic acid and its salts, dithiocarbamic acid and its salts, salts of xanthates, salts of ethylene diamine tetraacetic acid and salts of tetraphenylboronic acid.
 13. A composition according to claim 1, wherein said donor contains a nitrogen, oxygen, phosphorus or sulfur donor atom and an abstractable hydrogen atom bonded to a carbon or silicon atom alpha to said donor atom.
 14. A composition according to claim 1, wherein said donor comprises a tertiary amine containing an aromatic ring.
 15. A composition according to claim 14, wherein there is at least one electron-withdrawing group on said aromatic ring.
 16. A composition according to claim 1, wherein said donor contains one or more julolidinyl moieties.
 17. A composition according to claim 16, wherein said donor comprises 9-julolidine carboxyaldehyde.
 18. A composition according to claim 1, wherein said composition contains, for every 100 parts by weight of said monomer, about 0.005 to about 10 parts by weight each of said aryliodonium salt, said sensitizing compound and said donor.
 19. A composition according to claim 18, wherein said composition contains, for every 100 parts by weight of said monomer, about 0.1 to about 4 parts by weight each of said aryliodonium salt, said sensitizing compound and said donor.
 20. A composition according to claim 18, wherein said composition further comprises about 10% to 90% reinforcing and/or extending fillers, based on the total weight of said composition.
 21. A composition according to claim 20, in the form of a photocurable adhesive applied to an orthodontic bracket.
 22. A composition according to claim 21, wherein said sensitizing compound comprises camphorquinone and said donor comprises triethanolamine.
 23. A composition according to claim 20, in the form of a photocurable composite suitable for use as a dental restorative.
 24. A composition according to claim 1, in the form of a photocurable or photocured image-bearing coating atop a support.
 25. A composition according to claim 24, wherein said coating and said support comprise a printing plate-in which, before photopolymerization, said sensitizing compound comprises a ketene, ketocoumarin, aminoarylketone or p-substituted aminostyryl ketone compound.
 26. A composition according to claim 25, wherein said donor comprises 9-julolidine carboxaldehyde.
 27. An addition-polymerizable composition comprising:a) free-radically-polymerizable monomer, and b) photoinitiator system, soluble in said monomer, comprising 0.1 to 4 parts each ofi) diphenyliodonium metal complex salt, ii) ketone, ketocoumarin, aminoarylketone or p-substituted aminostyryl ketone sensitizing compound capable of absorbing light somewhere within the range of wavelengths between about 400 and about 600 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and iii) electron donor compound containing a nitrogen donor atom and having an abstractable hydrogen atom bonded to a carbon or silicon atom alpha to said nitrogen atom,wherein said donor compound is different from said sensitizing compound and wherein the oxidation potential of said donor is greater than about 0.5 and less than or equal to about 1 volt vs. a saturated calomel electrode.
 28. A method for addition photopolymerization comprising the step of irradiating a composition with light having a wavelength between about 300 and about 1000 nanometers until said composition gels or hardens, said composition comprising:a) free-radically-polymerizable monomer, and b) photoinitiator system, soluble in said monomer, comprising photochemically effective amounts ofi) aryliodonium salt, ii) sensitizing compound capable of absorbing light somewhere within the range of wavelengths between about 300 and about 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and iii) electron donor compound, said donor being different from said sensitizing compound and ..Wherein the oxidation potential of said donor is greater than zero vs. a saturated Calomel electrode.
 29. An addition-polymerizable dental adhesive composition comprising:a) free-radically-polymerizable monomer, and b) photoinitiator system, soluble in said monomer, comprising photochemically effective amounts ofi) aryliodonium salt, ii) sensitizing compound capable of absorbing light within the range of wavelengths between about 300 and about 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and iii) electron donor compound, said donor being different from said sensitizing compound and wherein the oxidation potential of said donor is greater than and less than or equal to vs. a saturated calomel electrode.
 30. The composition of claim 29, whereini) the aryliodonium salt is the salt of diphenyl iodonium; ii) the sensitizing compound is an a-diketone having an extinction coefficient below about 1000 at the wavelength at which said composition is irradiated when photopolymerized; and iii) the electron donor compound is a tertiary amine containing an aromatic ring.
 31. An addition-polymerizable dental composite composition comprising:a) free-radically-polymerizable monomer, and b) photoinitiator system, soluble in said monomer, comprising photochemically effective amounts ofi) aryliodonium salt, ii) sensitizing compound capable of absorbing light within the range of wavelengths between about 300 and about 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and iii) electron donor compound, said donor being different from said sensitizing compound and wherein the oxidation potential of said donor is greater than and less than or equal to vs. a saturated calomel electrode.
 32. The composition of claim 31, whereini) the aryliodonium salt is the salt of diphenyl iodonium; ii) the sensitizing compound is an a-diketone having an extinction coefficient below about 1000 at the wavelength at which said composition is irradiated when photopolymerized; and iii) the electron donor compound is a tertiary amine containing an aromatic ring.
 33. An addition-polymerizable orthodontic adhesive composition comprising:a) free-radically-polymerizable monomer, and b) photoinitiator system, soluble in said monomer, comprising photochemically effective amounts ofi) aryliodonium salt, ii) sensitizing compound capable of absorbing light within the range of wavelengths between about 300 and about 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)-s-triazine, and iii) electron donor compound, said donor being different from said sensitizing compound and wherein the oxidation potential of said donor is greater than and less than or equal to vs. a saturated calomel electrode.
 34. The composition of claim 33, whereini) the aryliodonium salt is the salt of diphenyl iodonium; ii) the sensitizing compound is an a-diketone having an extinction coefficient below about 1000 at the wavelength at which said composition is irradiated when photopolymerized; and iii) the electron donor compound is a tertiary amine containing an aromatic ring.
 35. A photoinitiator system for initiating polymerization of an addition polymerizable composition comprising:i) atyliodonium salts, ii) sensitizing compound capable of absorbing light within the range of wavelengths between about 300 and about 1000 nanometers and capable of sensitizing 2-methyl-4,6-bis(trichloromethyl)s-triazine, and iii) an electron donor compound, said donor being different from said sensitizing compound wherein the oxidation potential of said donor is greater than zero and less than or equal to p-dimethoxybenzene vs. a saturated calomel electrode. 